Astronomers Found the Ghost of a Rare Giant Radio Galaxy

Astronomers in India have discovered a very unusual galaxy, and it’s dying. By now, in fact, it’s probably already dead.

The new galaxy, known as J021659-044920, is 9 billion light years away from Earth. That means it’s really old in cosmic terms (but not quite as old as the oldest object astronomers have ever found, a galaxy 13 billion light years away called UDFy-38135539). Viewed in the visible spectrum, J021659-044920spans about 100,000 light years from one edge to the other - but there’s much more there than meets the eye.

When Prathamesh Tamhane, a student at the Indian Institute of Science Education and Research, and his colleagues looked at J021659-044920 in the radio spectrum, they saw giant lobes of radio emissions, stretching 4 million light years from end to end. J021659-044920 is what astronomers call a giant radio galaxy, and it’s a rare find. Tamhane and his colleagues published their discovery in the journal Monthly Notices of the Royal Astronomical Society.

Like most galaxies, this one has a supermassive black hole at its heart. As the black hole’s incredible gravity draws material toward it, the doomed material falls inward in a spiral. The motion of all that charged material creates powerful electromagnetic forces, which can accelerate material away from the black hole at nearly the speed of light. Those jets of hot plasma, blasting away from the black hole in opposite directions, produce huge lobes of radio emissions that can span much greater distances than the galaxy’s visible light.

Astronomers call these “radio galaxies,” and smaller ones, less than a million light years across, are relatively common, in astronomical terms. The image above, for instance, is radio galaxy Hercules A, shown in a visible light image from the Hubble Space Telescope combined with a radio image from the Very Large Array in New Mexico. But only a few giant radio galaxies like J021659-044920 are much rarer, especially at such great distances, have ever been found. And this one is dying.

Image: J021659-044920. The red and yellow lobes are the galaxy’s radio lobes. The red spot in the centre is the visible galaxy. Prathamesh Tamhane/Yogesh Wadadekar.

When the black hole at the centre of a radio galaxy stops producing plasma jets, the radio lobes’ power source gets cut off, and they slowly fade away over the next few million years. Tamhane and his colleagues observed J021659-044920 at this final stage of its life, with its enormous radio lobes still there but beginning to fade. In fact, because J021659-044920 is so far away, its radio lobes probably faded out long ago, but their light is only now reaching Earth.